#include <string.h>
char* insert(char* s1, char* s2, int n) {
    int i,l1,l2;
    l1 = strlen(s1);
    l2 = strlen(s2);
    for (i=l1-1; i>n-1; --i) {
        s1[i + l2] = s1[i];
    }
    for (i=n; i<n+l2; ++i) {
        s1[i] = s2[i-n];
    }
    s1[l1 + l2] = 0;
    return s1;
}

